Production of a titania-pigmented polyamide

ABSTRACT

Disclosed is a method for producing a polyamide containing titanium dioxide pigments. Said method is characterized by the fact that titanium dioxide pigments are dispersed in an initial mixture containing water and caprolactam by means of a device comprising a dispersion chamber, a disk-shaped rotor that is disposed inside said dispersion chamber, a stator that is provided with radial openings, is arranged within the dispersion zone of the dispersion chamber, and is connected to the rotor, a substance inlet located on each side of the rotor, preferably with a respective axial duct section such that the two flows of substance are combined in the peripheral edge area of the rotor disk, and a product outlet which is placed on the outer edge of the dispersion zone of the dispersion chamber, by feeding the titanium dioxide pigments to the dispersion chamber through one of said substance inlets while feeding the initial mixture containing water and caprolactam to the dispersion chamber through the other of said substance inlets, whereby a product mixture containing water, caprolactam, and the used titanium dioxide pigment is obtained via the product outlet, and the product mixture is polymerized so as to obtain a polyamide containing titanium dioxide pigments. Also disclosed are polyamides obtained according to the inventive method and the use of such polyamides as a masterbatch.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/011574, filed Oct. 18, 2003, which claims priority fromGerman Patent Application No. DE 102 49 797.4, filed Oct. 24, 2002.

The present invention relates to a process for preparing a polyamidecontaining titania (titanium dioxide) pigments, polyamides obtainable bysuch a process and the use of such polyamides as a masterbatch forcoloration or delustering polymers.

The use of titanium dioxide to deluster or color polyamides is commonknowledge.

It is generally desirable for this use that the titanium dioxide bepresent in a very finely and uniformly divided form and the polyamidesobtained by coloration or delustering with titanium dioxide have verylow abrasivity due in particular to the titanium dioxide pigments in thecourse of further processing, for example spinning.

Various techniques are known for introducing titanium dioxide pigmentsinto a polyamide.

For instance, Fourné, Synthetische Fasern, Carl Hanser Verlag,Munich-Vienna, 1995, pages 629-630, chapters 6.8 and 6.8.1, describesthe addition of titanium dioxide at the start of the polymerization orthe addition of a masterbatch, ie a polyamide having a high titaniumdioxide content, to the substantially titanium dioxide-free main streamto be polymerized, for the purposes of delustering polyamides.

EP-A-70 452 discloses kneading the titanium dioxide into caprolactamfollowed by kneading with water, slurrying of the resultant paste withwater, sedimenting of the coarse titanium dioxide particles anddecanting from the sedimented fraction.

These processes for producing titania-pigmented polymers are stillunsatisfactory and in need of further improvement.

It is an object of the present invention to provide a process wherebytitania-pigmented polyamides having improved properties are obtained ina technically simple and economical manner.

We have found that this object is achieved by a process for producing apolyamide which contains titanium dioxide pigments, which comprises

dispersing titanium dioxide pigments in a starting mixture containingwater and caprolactam by means of an apparatus,

said apparatus comprising

a dispersing chamber,

a disk-shaped rotor disposed in said dispersing chamber,

a stator which has radial openings and is disposed in conjunction withsaid rotor in the dispersing zone of said dispersing chamber,

a product inlet on each side of said rotor, each product inletpreferably having an axial channel section such that the confluence ofthe two product streams is disposed in the outer peripheral region ofthe rotor disk, and

a product outlet at the outer periphery of said dispersing zone of saiddispersing chamber,

by feeding the titanium dioxide pigments through one of said productinlets and the starting mixture, containing water and caprolactam,through the other one of said product inlets, to said dispersingchamber, and obtaining a product mixture, containing water, caprolactamand titanium dioxide used, via said product outlet,

and

polymerizing said product mixture to a polyamide containing titaniumdioxide pigments.

The present invention further provides polyamides obtainable by thisprocess and for the use of such polyamides as a masterbatch fordelustering or coloration of a polymer.

The process of the present invention relates to the production of apolyamide.

Polyamides are herein to be understood as being homopolymers,copolymers, blends and grafts of synthetic long-chain polyamides havingrecurring amide groups in the polymer main chain as an essentialconstituent. Examples of such polyamides are nylon-6 (polycaprolactam),nylon-6,6 (polyhexamethyleneadipamide),nylon-4,6(polytetramethyleneadipamide), nylon-6,10(polyhexamethylenesebacamide), nylon-7 (polyenantholactam), nylon-11(poly-undecanolactam), nylon-12 (polydodecanolactam). As well aspolyamides known by the generic name of nylon, polyamides furtherinclude the aramids (aromatic polyamides), such aspoly-meta-phenyleneisophthalamide (NOMEX® fiber, U.S. Pat. No.3,287,324) or poly-para-phenyleneterephthalamide (KEVLAR® fiber, U.S.Pat. No. 3,671,542).

Polyamides can in principle be prepared by two methods.

In a polymerization from dicarboxylic acids and diamines and also in apolymerization from amino acids or their derivatives, such asaminocarbonitriles, aminocarboxamides, aminocarboxylate esters oraminocarboxylate salts, the amino and carboxyl end groups of thestarting monomers or starting oligomers react with one another to forman amide group and water. The water can subsequently be removed from thepolymer. In a polymerization from carboxamides, the amino and amide endgroups of the starting monomers or starting oligomers react with oneanother to form an amide group and ammonia. The ammonia can subsequentlybe removed from the polymer. This polymerization reaction is customarilyknown as a polycondensation.

A polymerization from lactams as starting monomers or starting oligomersis customarily known as a polyaddition.

Such polyamides are obtainable by conventional processes, as describedfor example in DE-A-14 95 198, DE-A-25 58 480, EP-A-129 196 or in:Polymerization Processes, Interscience, New York, 1977, pages 424-467,especially pages 444-446, from monomers selected from the groupconsisting of lactams, omega-aminocarboxylic acids,omega-aminocarbonitriles, omega-aminocarboxamides,omega-aminocarboxylate salts, omega-aminocarboxylate esters, equimolarmixtures of diamines and dicarboxylic acids, dicarboxylic acid/diaminesalts, dinitriles and diamines or mixtures thereof.

Useful monomers include

monomers or oligomers of a C₂ to C₂₀, preferably C₂ to C₁₈,arylaliphatic or, preferably, aliphatic lactam such as enantholactam,undecanolactam, dodecanolactam or caprolactam,

monomers or oligomers of C₂ to C₂₀, preferably C₃ to C₁₈,aminocarboxylic acids such as 6-aminocaproic acid or 11-aminoundecanoicacid, and dimers, trimers, tetramers, pentamers or hexamers thereof, andsalts thereof such as alkali metal salts, for example lithium, sodium orpotassium salts,

C₂ to C₂₀, preferably C₃ to C₈, aminocarbonitriles such as6-aminocapronitrile or 11-aminoundecanonitrile,

monomers or oligomers of C₂ to C₂₀ amino acid amides such as6-aminocapramide or 11-aminoundecaomide, and dimers, trimers, tetramers,pentamers or hexamers thereof,

esters, preferably C₁-C₄ alkyl esters, such as methyl, ethyl, n-propyl,i-propyl, n-butyl, i-butyl or s-butyl esters, of C₂ to C₂₀, preferablyC₃ to C₁₈, aminocarboxylic acids, such as 6-aminocaproic acid esters,for example methyl 6-aminocaproate, or 11-aminoundecanoic acid esters,for example methyl 11-aminoundecanoate,

monomers or oligomers of a C₂ to C₂₀, preferably C₂ to C₁₂,alkyldiamine, such as tetramethylenediamine or, preferably,hexamethylenediamine,

with a C₂ to C₂₀, preferably C₂ to C₁₄, aliphatic dicarboxylic acid ormono-or dinitriles thereof, such as sebacic acid, dodecanedioic acid,adipic acid, sebacic acid dinitrile, decanoic acid dinitrile oradiponitrile,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₂ to C₂₀, preferably C₂ to C₁₂,alkyldiamine, such as tetramethylenediamine or, preferably,hexamethylenediamine,

with a C₈ to C₂₀, preferably C₈ to C₁₂, aromatic dicarboxylic acid orderivatives thereof, for example chlorides, such asnaphthalene-2,6-dicarboxylic acid, preferably isophthalic acid orterephthalic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₂ to C₂₀, preferably C₂ to C₁₂,alkyldiamine, such as tetramethylenediamine or, preferably,hexamethylenediamine,

with a C₉ to C₂₀, preferably C₉ to C₁₈, arylaliphatic dicarboxylic acidor derivatives thereof, for example chlorides, such as o-, m-orp-phenylenediacetic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₆ to C₂₀, preferably C₆ to C₁₀, aromaticdiamine, such as m-or p-phenylenediamine,

with a C₂ to C₂₀, preferably C₂ to C₁₄, aliphatic dicarboxylic acid ormono-or dinitriles thereof, such as sebacic acid, dodecanedioic acid,adipic acid, sebacic acid dinitrile, decanoic acid dinitrile oradiponitrile,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₆ to C₂₀, preferably C₆ to C₁₀, aromaticdiamine, such as m-or p-phenylenediamine,

with a C₈ to C₂₀, preferably C₈ to C₁₂, aromatic dicarboxylic acid orderivatives thereof, for example chlorides, such asnaphthalene-2,6-dicarboxylic acid, preferably isophthalic acid orterephthalic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₆ to C₂₀, preferably C₆ to C₁₀, aromaticdiamine, such as m-or p-phenylenediamine,

with a C₉ to C₂₀, preferably C₉ to C₁₈, arylaliphatic dicarboxylic acidor derivatives thereof, for example chlorides, such as o-, m-orp-phenylenediacetic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₇ to C₂₀, preferably C₈ to C₁₈,arylaliphatic diamine, such as m-or p-xylylenediamine,

with a C₂ to C₂₀, preferably C₂ to C₁₄, aliphatic dicarboxylic acid ormono-or dinitriles thereof, such as sebacic acid, dodecanedioic acid,adipic acid, sebacic acid dinitrile, decanoic acid dinitrile oradiponitrile,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₇ to C₂₀, preferably C₈ to C₁₈,arylaliphatic diamine, such as m-or p-xylylenediamine,

with a C₆ to C₂₀, preferably C₆ to C₁₀, aromatic dicarboxylic acid orderivatives thereof, for example chlorides, such asnaphthalene-2,6-dicarboxylic acid, preferably isophthalic acid orterephthalic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

monomers or oligomers of a C₇ to C₂₀, preferably C₈ to C₁₈,arylaliphatic diamine, such as m-or p-xylylenediamine,

with a C₉ to C₂₀, preferably C₉ to C₁₈, arylaliphatic dicarboxylic acidor derivatives thereof, for example chlorides, such as o-, m-orp-phenylenediacetic acid,

and dimers, trimers, tetramers, pentamers or hexamers thereof,

and homopolymers, copolymers, mixtures and grafts of such startingmonomers or starting oligomers.

In a preferred embodiment, the lactam used is caprolactam, the diamineused is tetramethylenediamine, hexamethylenediamine or their mixturesand the dicarboxylic acid used is adipic acid, sebacic acid,dodecanedioic acid, terephthalic acid, isophthalic acid or mixturesthereof. Particular preference is given to the lactam being caprolactam,the diamine being hexamethylenediamine and the dicarboxylic acid beingadipic acid or terephthalic acid or their mixtures.

Particular preference is given to those starting monomers or oligomerswhich on polymerization lead to the polyamides nylon-6, nylon-6,6,nylon-4,6, nylon-6,10, nylon-6,12, nylon-7, nylon-11 or nylon-12 or thearamids poly-meta-phenyleneisophthalamide orpoly-para-phenyleneterephthalamide, especially to nylon-6 or nylon-6,6.

In a preferred embodiment, the polyamides may be prepared using one ormore chain regulators. Useful chain regulators advantageously includecompounds having one or more, such as two, three or four, preferably twoin the case of systems in the form of fibers, amino groups reactive inpolyamide formation or one or more, such as two, three or four,preferably two in the case of systems in the form of fibers, carboxylgroups reactive in polyamide formation.

The first case provides polyamides wherein said monomers used forpreparing said polyamide have a higher number of amine groups, or theirequivalents, used for forming said polymer chain than carboxylic acidgroups, or their equivalents, used for forming said polymer chain.

The second case provides polyamides wherein said monomers used forpreparing said polyamide have a higher number of carboxylic acid groups,or their equivalents, used for forming said polymer chain than aminegroups, or their equivalents, used for forming said polymer chain.

Useful chain regulators advantageously include monocarboxylic acids,such as alkanecarboxylic acids, for example acetic acid, propionic acid,such as benzene-or naphthalene-monocarboxylic acid, for example benzoicacid, dicarboxylic acids, such as C₄-C₁₀-alkanedicarboxylic acid, forexample adipic acid, azelaic acid, sebacic acid, dodecanedioic acid,C₅-C₈-cycloalkanedicarboxylic acids, for examplecyclohexane-1,4-dicarboxylic acid, benzene-or naphthalenedicarboxylicacid, for example terephthalic acid, isophthalic acid,naphthalene-2,6-dicarboxylic acid, C₂ to C₂₀, preferably C₂ to C₁₂,alkylamines, such as cyclohexylamine, C₆ to C₂₀, preferably C₆ to C₁₀,aromatic monoamines, such as aniline, or C₇ to C₂₀, preferably C₈ toC₁₈, arylaliphatic monoamines, such as benzylamine, diamines, such asC₄-C₁₀-alkanediamines, for example hexamethylenediamine.

The chain regulators may be unsubstituted or substituted, for example byaliphatic groups, preferably C₁-C₈-alkyl groups, such as methyl, ethyl,i-propyl, n-propyl, n-butyl, i-butyl, s-butyl, n-pentyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, OH, ═O, C₁-C₈-alkoxy, COOH,C₂-C₆-carbalkoxy, C₁-C₁₀-acyloxy, or C₁-C₈-alkylamino, sulfonic acid orsalts thereof, such as alkali or alkaline earth metal salts, cyano orhalogens, such as fluorine, chlorine, bromine.

Examples of substituted chain regulators are sulfoisophthalic acid andalkali or alkaline earth metal salts thereof, such as lithium, sodium orpotassium salts, sulfoisophthalic esters, for example withC₁-C₁₆-alkanols, or sulfoisophthalic acid mono-or diamides, especiallywith monomers suitable for forming polyamides and bearing at least oneamine group, such as hexamethylenediamine or 6-aminocaproic acid.

A chain regulator may advantageously be used in amounts of not less than0.01 mol %, preferably not less than 0.05 mol %, especially not lessthan 0.2 mol %, based on 1 mol of acid amide groups of the polyamide.

A chain regulator may advantageously be used in amounts of not more than1.0 mol %, preferably not more than 0.6 mol %, especially not more than0.5 mol %, based on 1 mol of acid amide groups of the polyamide.

Advantageously, the polyamide may contain a sterically hinderedpiperidine derivative attached to the polymer chain by chemical bonding.The polyamide may also contain mixtures of such sterically hinderedpiperidine derivatives.

Preferred sterically hindered piperidine derivatives are those of theformula

where

-   R¹ is a functional group capable of amide formation with the polymer    chain of the polyamide, preferably a group —(NH)R⁵, in which R⁵ is    hydrogen or C₁-C₈ alkyl, or a carboxyl group, or a carboxyl    derivative, or a group —(CH₂)_(x)(NH)R⁵, in which x is 1 to 6 and R⁵    is hydrogen or C₁-C₈ alkyl, or a group —(CH₂)_(y)COOH, in which y is    1 to 6, or a —(CH₂)_(y)COOH acid derivative, in which y is 1 to 6,    -   especially a group —NH₂,-   R² is an alkyl group, preferably a C₁-C₄ alkyl group such as methyl,    ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,    -   especially a methyl group,-   R³ is hydrogen, C₁-C₄ alkyl or O—R⁴, in which R⁴ is hydrogen or    C₁-C₇ alkyl,    -   R³ being hydrogen in particular.

In such compounds, steric hindrance usually prevents the tertiary aminogroups, and especially the secondary amino groups, of the piperidinering system from reacting.

A particularly preferred sterically hindered piperidine derivative is4-amino-2,2,6,6-tetramethylpiperidine.

The sterically hindered piperidine derivative may advantageously be usedin amounts of not less than 0.01 mol %, preferably not less than 0.05mol %, especially not less than 0.1 mol %, based on 1 mole of acid amidegroups of the polyamide.

The sterically hindered piperidine derivative may advantageously be usedin amounts of not more than 0.8 mol %, preferably not more than 0.6 mol%, especially not more than 0.4 mol %, based on 1 mol of acid amidegroups of the polyamide.

Such polyamides which contain a sterically hindered piperidinederivative attached to the polymer chain by chemical bonding, andprocesses for the preparation of said polyamides are described forexample in WO 95/28443, WO 97/05189, WO 98/50610, WO 99/46323, WO99/48949, EP-A-822 275, EP-A-843 696 and the German applications10030515.6, 10030512.1 and 10058291.5.

The process of the present invention relates to the production ofpolyamide pigmented with titanium dioxide.

The titanium dioxide may be used in any possible crystal form, such asanatase, rutile or brookite or mixtures thereof. Preference is given toanatase or rutile or mixtures thereof, especially anatase, althoughsynthetic anatase may customarily contain small amounts of rutile, forexample up to 5% by weight based on the total weight, as a result of theprocess used to produce it.

The specific BET surface area of the titanium dioxide is not criticalper se, although as the BET surface area increases the reactivity oftitanium dioxide, like the chalking action, generally increases and asthe BET surface area decreases the average mean particle size generallyincreases.

For a titanium dioxide to be useful as a delusterant or colorant itshould have not only a low chalking action but also a low particle size.

The best BET surface area for the particular polyamide and theparticular application is easily determined by a few simple preliminarytests.

To reduce the chalking action of titanium dioxide, ie to reduce thephotochemical activity with regard to any decomposition of thesurrounding polyamide matrix, the titanium dioxide may be coated withcompounds of other elements, for example the oxides, advantageously ofmanganese, iron, antimony, silicon or aluminum.

Such titanium dioxide pigments and their manufacture are generally knownand suitable titanium dioxide pigments are commercially available.

According to the invention, titanium dioxide pigments are dispersed in astarting mixture containing water and caprolactam.

Advantageously, the starting mixture containing water and caprolactamshould contain the water and caprolactam in a ratio in the range from50/50 to 99/1, preferably in the range from 80/20 to 97/3, andespecially in the range from 93/7 to 95/5 mass/mass.

The starting mixture may advantageously include a dispersing assistant.

Useful dispersing assistants include protective colloids and/oremulsifiers as described for example in Houben-Weyl, Methoden derOrganischen Chemie, Vol. XIV/1, Makromolekulare Stoffe,Georg-Thieme-Verlag, Stuttgart, 1961, pp. 411-420 and pp. 192-208, morepreferably Na₂HPO₄ (eg disodium hydrogenphosphate—12 Hydrate; Chem.Werke Budenheim) and also sodium salts of modified polyacrylic acidshaving an average molecular weight of about 4000 (eg Sokalan PA 20 PN;BASF) or mixtures thereof.

The dispersing assistant may preferably be used in amounts from 0.01 to5.0% and especially from 0.1 to 1.0% by weight, based on startingmixture.

The starting mixture may in a preferred embodiment, as well ascaprolactam, contain further of the abovementioned polyamide-formingmonomers, chain regulators or piperidine derivatives or mixturesthereof.

In a further preferred embodiment, the starting mixture containscaprolactam as sole polyamide-forming monomer. In a particularlypreferred embodiment, the starting mixture contains none of thepiperidine derivatives mentioned. In a particularly preferredembodiment, the starting mixture contains none of the chain regulatorsmentioned.

The process of the present invention may utilize titanium dioxidepigment and the starting mixture in a ratio in the range from 1/99 to50/50, preferably in the range from 10/90 to 40/60, and especially inthe range from 15/85 to 25/75 mass/mass.

According to the invention, titanium dioxide pigments are dispersed inthe starting mixture by means of an apparatus,

said apparatus comprising

a dispersing chamber,

a disk-shaped rotor disposed in said dispersing chamber,

a stator which has radial openings and is disposed in conjunction withsaid rotor in the dispersing zone of said dispersing chamber,

a product inlet on each side of said rotor, each product inletpreferably having an axial channel section such that the confluence ofthe two product streams is disposed in the outer peripheral region ofthe rotor disk, and

a product outlet at the outer periphery of said dispersing zone of saiddispersing chamber.

An apparatus of this kind is known per se. Preferred embodiments ofsuitable apparatus are described in WO 92/21436 and especially preferredembodiments of suitable apparatus in WO 01/87474, the content of bothdocuments being incorporated herein by reference.

Suitable apparatus is commercially available, for example the TDS orpreferably Conti-TDS machines from Ystral GmbHMaschinenbau+Processtechnik, Germany.

A product mixture is obtained according to the invention by feeding thetitanium dioxide pigments through one of said product inlets and thestarting mixture, containing water and caprolactam, through the otherone of said product inlets, to said dispersing chamber, and obtaining aproduct mixture, containing water, caprolactam and the titanium dioxidepigment used, via said product outlet.

The best operating parameters for the particular type of machine and theparticular product composition desired are easily determined by a fewsimple preliminary tests.

According to the invention, the product mixture is polymerized to apolyamide containing titanium dioxide pigments.

The polymerization may advantageously be carried out according to one ofthe processes mentioned above for the production of polyamides.

In an advantageous embodiment, the product mixture may be stored priorto the polymerization, for example in a settling vessel, in which casedelay times in the range from 1 to 100 hours and especially in the rangefrom 10 to 50 hours are suitable, in order that any comparatively coarsetitanium dioxide pigments present in the product mixture may be allowedto sediment out.

After sedimenting, the mixture destined for the polymerization can betaken off from the surface, such as by pumping off. The sediment mayadvantageously be returned into the dispersing stage. In a furtherpreferred embodiment, the polymerization may be preceded by reducing thewater content of the product mixture, especially after the sedimentationof the comparatively coarse titanium dioxide pigments, in a conventionalmanner, as by evaporation. Furthermore, in a preferred embodiment, themonomer content of the product mixture may be increased prior to thepolymerization, by adding further monomer, such as caprolactam. Theremoval of water and the addition of further monomer may be combined, inwhich case the water may be removed before, after or during, preferablybefore, the monomer is added.

In a preferred embodiment, the levels of titanium dioxide,polyamide-forming components and water may be set such that the productobtained is a titania-pigmented polyamide in which the titania contentis in the range from 1 to 50%, preferably in the range from 5 to 45% andespecially in the range from 8 to 40% by weight, based on total weight.

The polyamides obtainable by the process according to the presentinvention may advantageously be used as a masterbatch for delustering orcoloration of a polymer. What is contemplated here in particular is theaddition of the masterbatch in molten form to a main stream whichcontains polymer-forming monomers, such as caprolactam in the case ofnylon-6, during the polymerization. It is similarly preferable to addthe masterbatch to a polymer which differs from that of the masterbatch,for example by coextrusion or mixing in as a melt.

In principle, the titania-pigmented polyamides of the present inventionmay be incorporated into polymers by methods known per se for theincorporation of titania-pigmented polyamides into polymers.

The process of the present invention has the advantage that the titaniumdioxide pigments present in the polyamide have a mean average particlesize <1.2 μm, as measured by optical microscopy on thin sections, andhence are smaller than the particle size in polyamides produced by theprior art.

Further advantages of the process according to the present invention maybe discerned from the examples.

EXAMPLES Production of Masterbatches Inventive Example 1

A water ring pump (Conti-TDS, from Ystral GmbH Maschinenbau undProcesstechnik, Germany), was used to convert 1 000 kg/h of titaniumdioxide (Hombitan LOCR-SM, from Sachtleben Chemie GmbH, Duisburg), and 4100 kg/h of a mixture containing 93.8% of water, 6.0% by weight ofcaprolactam and 0.2% of dispersant (Sokalan PA 20 PN, from BASF AG), toa suspension over 2 h and transfer it into a stirred vessel andrecirculate it therein by means of a ring line.

The suspension was then transferred into a settling vessel and thecomparatively coarse titanium dioxide particles were allowed to sedimentfor 48 hours.

3.5 m³ of suspension were then pumped off the surface of the settlingvessel into a stock reservoir vessel. 5 m³ from the stock reservoirvessel were added over 5 h to 2 300 l of liquid caprolactam at atemperature of 125° C. and a pressure of 0.5 bar, while water introducedwith the suspension evaporated. As a result, the temperature of the meltrose as high as 152° C.

Thereafter, the suspension was transferred into a second, Diphyl-heatedvessel and heated to 257° C. at a constant vessel pressure of 6 bar byfurther evaporation of water.

The pressure was then lowered from 6 bar to 0.03 bar over 35 min.

After nitrogen blanketing, the melt was fed to an underwater pelletizerand the pellets were dried.

Inventive Example 2

Inventive example 1 was repeated except that 2 m³ of titania-containingsuspension were used instead of 5 m³ and 2 300 l of caprolactam insteadof 3 200 l.

Comparative Example

A titania-containing polyamide was produced as per the example ofEP-A-070 452.

Processing of Masterbatches

a) Filtration

The titania-containing polyamides obtained in inventive example 1,inventive example 2 and the comparative example were blended withcaprolactam and polymerized as per EP-A-070452 such that a polyamidehaving a titanium dioxide content of 1.6% by weight, based on totalweight, was obtained. The polyamides obtained by masterbatch as perinventive examples 1 and 2 are hereinafter referred to as polyamides 1and 2 respectively, while the polyamide obtained using the masterbatchas per the comparative example will be referred to as the comparativepolyamide.

The mixtures were each filtered six times in the melt at 262° C. at athroughput of 3.7 g/min through a nonwoven filter having an average meanpore size of 10 μm in the course of 12 hours.

The average mean pressure build-up, determined as a difference betweenthe final pressure and the initial pressure upstream of the filter,divided by the amount put through, was about 7 bar/kg for each ofpolyamides 1 and 2, while the pressure build-up in the case of thecomparative polyamide was 10 bar/kg.

b) Abrasion

Polyamides 1 and 2 and the comparative polyamide were each used to spina 44 dtex 12 filament yarn.

50 km of the yarn were pulled twice over a sheet of copper (curvature:90°, pulling force: 2 cN; speed: 150 m/min).

The yarn formed from polyamides 1 and 2 had an average abrasion of 3.8mg/100 km of yarn, while the yarn formed from the comparative polyamidehad an abrasion of 4.2 mg/100 km.

1. A process for producing a polyamide with titanium dioxide pigments,comprising: dispersing the titanium dioxide pigments in a mixturecontaining water and caprolactam with an apparatus, said apparatuscomprising a dispersing chamber, a disk-shaped rotor disposed in saiddispersing chamber, a stator which has radial openings and is disposedin conjunction with said rotor in a dispersing zone of said dispersingchamber, a product inlet on each side of said rotor, such that aconfluence of two product streams from each of the product inlets isdisposed in an outer peripheral region of the disk-shaped rotor, and aproduct outlet at the outer periphery of said dispersing zone of saiddispersing chamber, by feeding the titanium dioxide pigments through oneof said product inlets and the mixture, containing water andcaprolactam, through the other said product inlet to said dispersingchamber, and obtaining a product mixture, containing water, caprolactamand the titanium dioxide pigment from said product outlet, andpolymerizing said product mixture to form the polyamide containingtitanium dioxide pigments and wherein water is removed from the productmixture before or during the polymerization, blending said productmixture with additional caprolactam followed by polymerizing theadditional caprolactam to form a masterbatch and filtering themasterbatch in a melt through a filter, wherein an average mean pressurebuild-up of the polymerized masterbatch, determined as a differencebetween a final pressure and an initial pressure upstream of the filter,divided by an amount put through, is about 7 bar/kg.
 2. The process ofclaim 1, wherein the mixture containing water and caprolactam furthercomprises a dispersing assistant.
 3. The process of claim 1, whereinadditional caprolactam is added to the product mixture before or duringthe polymerization.
 4. The process of claim 1, wherein the productinlets have an axial channel section.
 5. The process of claim 1, whereinthe titanium dioxide pigments have a mean average particle size of lessthan 1-2 microns as measured by optical microscopy.
 6. The process ofclaim 1, wherein the mixture containing water and caprolactam has aweight ratio of water:caprolactam from 1:1 to 99:1.
 7. The process ofclaim 6, wherein the ratio of water:caprolactam is from 4:1 to 97:3. 8.The process of claim 1, wherein the mixture containing water andcaprolactam and the titanium dioxide pigments is added to each of theproduct inlets in a weight ratio of pigment:mixture from 1:99 to 1:1. 9.The process of claim 8, wherein the ratio of pigment:mixture is from15:85 to 1:3.
 10. The polyamide containing titanium dioxide pigmentsobtained by the process of claim 1 wherein an amount of titanium dioxidepigments in the product mixture ranges from 1 to 50% based on a totalweight of the product mixture, and wherein the titanium dioxide pigmentshave a mean average particle size of less than 1.2 microns.
 11. Thepolyamide of claim 10, wherein the amount of titanium dioxide pigmentsin the product mixture ranges from 5 to 45% by weight based on total thetotal weight of the product mixture.
 12. The polyamide of claim 10,wherein the amount of titanium dioxide pigments in the product mixtureranges from 8 to 40% by weight based on total the total weight of theproduct mixture.
 13. The polyamide of claim 10, wherein the productmixture comprises a dispersing assistant.
 14. The polyamide of claim 13,wherein an amount of the dispersing assistant ranges from 0.1 to 1.0% byweight.
 15. The polyamide of claim 13, wherein the amount of water is93.8% by weight, the amount of caprolactam is 6.0% by weight, and theamount of dispersing assistant is 0.2% by weight.